(i) Field of the Invention
The present invention relates to a black dyestuff for heat-sensitive transfer record and a heat-sensitive transfer sheet containing the dyestuff.
Furthermore, the present invention relates to a blue dyestuff for heat-sensitive transfer record, particularly a blue dyestuff which is useful as a blue component of a blend black dyestuff for use in black record.
(ii) Description of the Prior Art
A heat-sensitive transfer system comprises superposing a heat transfer sheet coated with an ink containing a dyestuff for sublimation heat transfer upon an image receiving paper, and then pressing a thermal head against them to sublimate the dyestuff and to thereby transfer it to the image receiving paper, whereby a full color copy having a good gradation can be obtained.
The first feature of the heat-sensitive record system is that the thermal head is mainly used as a hard device and maintenance and reliability are excellent. The second feature of the heat-sensitive record system is that since a mixed color image can be obtained on a molecular level, there can be obtained a record similar to a silver salt color photograph which has excellent gradation and reproducibility.
In addition, this system is applied as a full color record system for computer graphics, a plate making system, video movies, a still video system, prepaid cards and the like, and hence much attention is paid to the above-mentioned heat-sensitive record system.
More specifically, the system of the present invention comprises heating a transfer sheet coated with a sublimable dyestuff by a thermal head controlled in accordance with a color signal of a still image to transfer the dyestuff to an image receiving paper, thereby recording the same thereon. As the dyestuffs for the transfer sheet, there are used three primary colors of yellow, magenta and cyan. Furthermore, in order to obtain the precise image in business and the like, four primary colors of the above-mentioned three colors and a black color can be used for the sake of the effect of light and shade.
Requirements for the black heat-sensitive transfer sheet are as follows.
(1) In a solvent-of an ink and a binder resin which are used at the time of the coating of the transfer sheet, the dyestuff should be soluble and compatible.
(2) Heat energy at the time of the sublimation transfer of the dyestuff from the transfer sheet to an image receiving sheet should be as low as possible, and an image having a color value should be transferred to the image receiving sheet.
(3) A transmittance curve of the transferred black image should be flat in the wave length range of 400 to 700 nm.
(4) The dyestuff in the transfer sheet should not recrystallize even in a high-temperature or high-humidity state.
(5) The transferred image should be excellent in light resistance and shelf stability.
(6) The graduations of matrix dyestuffs for the black color are similar to each other.
Among these requirements, the particularly desired items are "the dyestuff should be soluble in a solvent of an ink and should be compatible with a binder resin" in the paragraph (1), and the requirements of the paragraphs (3) and (6).
That is, in the heat-sensitive transfer system, the dyestuff sublimates in the molecular state,. and therefore the dyestuff is required to be completely dissolved in the binder resin on the transfer sheet. If the dyestuff which is not dissolved in the binder resin on the transfer. Sheet is transferred, any uniform image cannot be obtained and an image concentration is also low, so that a product value noticeably deteriorates.
Development has been continued so as to meet all of the above-mentioned requirements (1) to (6) necessary for the transfer sheet, and nowadays, it has been attempted to meet the requirement (3) by blending a plurality of sublimable dyestuffs. In addition, in order to obtain a good gradation, i.e., in order to meet the requirement (6), some matrix dyestuffs having the mutually similar gradations have been investigated as blend dyestuffs for the black color. However, any dyestuff for black transfer record which can meet all of these requirements (1) to (6) has not been developed, and further development is desired.
On the other hand, azo-based blue dyestuffs which have been heretofore suggested usually have a high molar absorptivity coefficient, and therefore most of the blue dyestuffs latently have a dyestuff concentration required for the transfer of the image. However, the blue dyestuff is less soluble in the solvent and less compatible with the binder resin, with the result that the blue dyestuff crystallizes on the transfer sheet and the heat transfer efficiency of the blue dyestuff noticeably deteriorates. Additionally, the adhesion of the transferred blue dyestuff to the image receiving paper is poor, and the image on the image receiving paper, when touched, is soiled, so that the quality of the image itself is impaired. Moreover, the blue dyestuff is poor in durability and particularly-light resistance which are most important, and it often has a problem that it cannot withstand a long-term storage. The dyestuffs by which these problems can be solved are extremely limited.